WO2001048971A1 - Amelioration portant sur un annuleur d'echo de modem numerique - Google Patents

Amelioration portant sur un annuleur d'echo de modem numerique Download PDF

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Publication number
WO2001048971A1
WO2001048971A1 PCT/US1999/030934 US9930934W WO0148971A1 WO 2001048971 A1 WO2001048971 A1 WO 2001048971A1 US 9930934 W US9930934 W US 9930934W WO 0148971 A1 WO0148971 A1 WO 0148971A1
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WO
WIPO (PCT)
Prior art keywords
echo
pcm
signal
linear
modem
Prior art date
Application number
PCT/US1999/030934
Other languages
English (en)
Inventor
Zoran Mladenovic
Piyush Shankerbhai Patel
Original Assignee
Telogy Networks, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Telogy Networks, Inc. filed Critical Telogy Networks, Inc.
Priority to EP99967632A priority Critical patent/EP1157498A4/fr
Priority to JP2001548964A priority patent/JP2003531510A/ja
Priority to AU23884/00A priority patent/AU2388400A/en
Publication of WO2001048971A1 publication Critical patent/WO2001048971A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/14—Two-way operation using the same type of signal, i.e. duplex
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B3/00—Line transmission systems
    • H04B3/02—Details
    • H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
    • H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/38—Synchronous or start-stop systems, e.g. for Baudot code
    • H04L25/40—Transmitting circuits; Receiving circuits
    • H04L25/49—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems
    • H04L25/4917—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems using multilevel codes
    • H04L25/4927—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems using multilevel codes using levels matched to the quantisation levels of the channel

Definitions

  • the present invention relates to digital transmission systems. More specifically, the present invention relates to echo cancellation in transmission over digital modems.
  • a modem is connected through a hybrid circuit to the telephone network local loop.
  • a local loop connects the local hybrid circuit to the local central office.
  • the trunk network connects the local central office to the remote central office, which is then in turn connected to the remote hybrid circuit through a local loop at the remote end.
  • a hybrid circuit connects the modem or telephone two wire circuit to the local loop four wire circuit. Each circuit carries a signal in both direction, transmit and receive.
  • a typical telephone network employs a hybrid converter to connect the unidirectional four wire link from the PSTN to the local two wire loop.
  • the four wire circuit has one wire pair for the signal in each direction. The two wire pair must duplex the signal to carry both transmit and receive signals on a single wire pair.
  • Part of the transmitted signal from the modem or telephone can be reflected back to the transmitter by the hybrid circuit, resulting in the speaker hearing his or her own speech delayed as an echo. This is referred to as near end echo.
  • a far end echo can also occur when the transmit signal passes through the PSTN and is reflected back by the hybrid at the far end of the PSTN.
  • the modem 1 is connected through the hybrid circuit 2 to the telephone network local loop 3.
  • the hybrid circuitry 2 as well as D/A and A/D converters 4 and 5, are selected to provide maximum linearity and the minimum possible noise floor, so that the overall modem performance, including the echo canceler, is maximized.
  • the near end echo which presents the major impairment in an analog modem, is canceled to the level of background noise floor.
  • the far end echo is passed through the PSTN 6 and to some degree non-linearly distorted due to non-linearities present in the network. However, the level of far end echo is attenuated due to local loop attenuation.
  • digital modems are connected to the PSTN through digital Tl or El lines.
  • the near end echo is typically nonexistent, because there is no local four to two wire conversion.
  • the far end echo is more emphasized, because it is not attenuated by a local loop, since no local loop is present.
  • a modem echo canceler is typically implemented as a linear adaptive filter, which is used to adapt to the echo path. In theory, it is known that the filter convergence of a linear adaptive filter will be fastest when the input signal is uncorrelated. In an analog modem implementation, a typical near end echo loop is highly linear, and the input data to the echo canceler is uncorrelated (data-driven echo canceler) , to provide for the optimal convergence properties.
  • the far end echo path once it has returned to the near end, has passed through two PCM to linear and linear to PCM conversions, which present a significant source of non-linearities.
  • One of those conversions is performed within the digital modem, and the other one is performed in the remote central office as part of the D/A or A/D conversion in the hybrid circuitry.
  • the typical PSTN digital modem connection is subject to far end echo which cannot be completely canceled using conventional approaches, i.e., a linear adaptive filter.
  • the echo canceler synthesizes the far end echo which is then subtracted from the composite signal of the far end signal and echo.
  • SUMMARY OF THE INVENTION In order to improve the reduction of echo in a digital modem, the near end sample is taken after linear to PCM conversion and passed through a PCM to linear conversion before generation of the echo cancellation signal and recombination with the far end signal.
  • the echo cancellation will more closely match any far end echo due to the introduction of conversion distortions more closely resembling those created by the conversion of the signal during transmission through the network.
  • the conversions experienced by the signal path are nonlinear in nature due to the ⁇ -law or A-law conversions, and they present a major problem to a linear adaptive filter used in echo cancelers, since the filter is not able to cancel them.
  • the amount of echo rejection is limited to about 33 dB.
  • the present invention can improve echo rejection to about 36dB.
  • the current invention provides a transmitter modem configuration which introduces a linear to PCM conversion and a PCM to linear conversion in a conventional echo canceler line with a resultant improvement in the amount of echo rejection.
  • a mode -based echo cancellation system and method for increasing the amount of echo rejection in a digital modem This increase is achieved by passing modem signals from the linear to PCM conversion, through digital delay line and a PCM to linear conversion before passing to the echo canceler. The signal provided to the echo canceler is thus converted from linear and reverted back to linear, partially matching the conversion distortion of the far end echo path, minimizing far end echo path non-linearity with the echo cancellation signal.
  • the present invention improves echo rejection in a PSTN digital modem connection.
  • the present invention minimizes far end echo path non-linearity in an echo canceler line in a PSTN digital modem connection.
  • the present invention reduces memory requirements for the echo canceler delay line by after linear to PCM conversion storing samples at low computational cost in a PSTN digital modem connection.
  • Figure 1 is a diagram of a typical analog modem connection
  • Figure 2 is a diagram of a typical digital modem connection
  • Figure 3 is a diagram of an exemplary embodiment of a modem-based improved echo cancellation system according to the present invention.
  • a typical PSTN digital modem 10 includes a modem transmitter 11, a modem receiver 12, a linear to PCM converter 13 and an echo canceler 14.
  • the modem is connected to a central office 16 and trunk network 17 through a Tl or El line 14.
  • the signal is converted to analog at D/A converter 19 at the far end central office 18 and supplied to the far end local loop 22 through a hybrid 20 at the far end.
  • Far end central office 18, also includes A/D converter 21.
  • the signal from the far end modem 24 is transmitted through hybrid 26, and local loop 22 to hybrid 20.
  • the far end signal is then converted to digital in A/D converter 21 for transmission through the network 17. Part of the far end signal can leak through hybrid 20 and become an echo on the signal passing back to the near end receiver 12.
  • the far end signal and far end echo pass through PCM to linear converter 16. After passing through converter 16, the far end signal and echo are combined at summer 15 with the output from echo canceler 14. Because the original linear near end signal is provided to echo canceler 14, before any PCM conversion, the linear distortion introduced into the echo coming form the far end is not present in the echo canceler signal. Although the far end echo has been converted back to a linear signal, distortions are present due to the dual conversions of this signal during its path. The differences between the converted echo signal and non converted echo canceling signal can reduce the effectiveness of the echo cancellation.
  • FIG. 3 an improvement in the amount of echo rejection, over that attainable by the configuration illustrated in FIG. 2, is achieved by providing the near end signal to the echo canceler 14, after linear to PCM conversion 13 and through additional PCM to linear converter 25.
  • the dual conversion of the near end signal prior to the echo canceler introduces non-linearities into the echo cancellation signal to better match the far end echo when recombined at summer 15.
  • the dual conversion of the echo cancellation signal path reduces the effect of non-linearities in the far end echo path.
  • the distorted transmit modem samples are reconverted, i.e., following the linear to PCM conversion 13 the distorted samples are fed through a delay line to a PCM to linear converter 25 prior to providing the signal to the echo canceler 14.
  • the achievable rejection using the preferred embodiment is about 36dB, which is 3dB better than the prior art method, FIG. 2.
  • the delay necessary to match the network delay of the far end echo can be accomplished by the inclusion of a delay line between linear to PCM converter 13 and PCM to linear converter 25. By placing the delay line between these converters, the delay line stores a PCM signal, therefore requiring less memory than storage of the original linear signal.
  • the delay line could be placed between converter 25 and echo canceler 14, within the scope of the invention. This embodiment would achieve the increased echo reduction from the compensation for non-linearities discussed above but would not benefit from the reduced memory requirements of a PCM delay line.
  • Typical network delays are of the order of 10-100 ms. However, in the case of satellite links delays of up to 1.2 seconds could be encountered. In modern DSPs, this delay requires storage, usually represented by 16 or more bits of up to 9.6k words, assuming an 8 kHz sampling rate. Using the approach of the preferred embodiment, the converted data could be stored with two or more echo delay line samples in the same word. This would result in a significant reduction in memory requirements for a small increase in computational cost.

Landscapes

  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
  • Telephonic Communication Services (AREA)
  • Bidirectional Digital Transmission (AREA)

Abstract

L'invention concerne une amélioration portée sur un annuleur d'écho de modem (10) destiné aux réseaux de communications numériques. Les effets sur l'annulation d'écho de non-linéarité dans un trajet d'écho à distance d'un système téléphonique numérique sont réduits par introductions de non-linéarités dans le signal local avant la génération du signal d'annulation d'écho. Le signal local est échantillonné après la conversion linéaire-MIC (13). Une conversion MIC-linéaire (25) est ajoutée avant l'annuleur d'écho (14). La ligne de retard locale peut être fournie entre les convertisseurs pour retarder le signal en tant qu'échantillons MIC, ce qui permet de réduire la capacité de mémoire requise de ligne de retard.
PCT/US1999/030934 1999-12-27 1999-12-27 Amelioration portant sur un annuleur d'echo de modem numerique WO2001048971A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
EP99967632A EP1157498A4 (fr) 1999-12-27 1999-12-27 Amelioration portant sur un annuleur d'echo de modem numerique
JP2001548964A JP2003531510A (ja) 1999-12-27 1999-12-27 デジタルモデムエコーキャンセラーの改良
AU23884/00A AU2388400A (en) 1999-12-27 1999-12-27 Digital modem echo canceler enhancement

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US09/472,031 US6795405B1 (en) 1999-12-27 1999-12-27 Digital modem echo canceler enhancement
US09/472,031 1999-12-27

Publications (1)

Publication Number Publication Date
WO2001048971A1 true WO2001048971A1 (fr) 2001-07-05

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PCT/US1999/030934 WO2001048971A1 (fr) 1999-12-27 1999-12-27 Amelioration portant sur un annuleur d'echo de modem numerique

Country Status (5)

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US (1) US6795405B1 (fr)
EP (1) EP1157498A4 (fr)
JP (1) JP2003531510A (fr)
AU (1) AU2388400A (fr)
WO (1) WO2001048971A1 (fr)

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7260142B2 (en) * 2002-03-07 2007-08-21 Texas Instruments Incorporated Dynamic hybrid switching in a wireline modem
US7194024B2 (en) * 2003-01-29 2007-03-20 Texas Instruments Incorporated Method for adaptive hybrid selection during ADSL modem training
TWI774575B (zh) * 2021-10-04 2022-08-11 瑞昱半導體股份有限公司 數位至類比轉換校正裝置

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5247512A (en) * 1991-08-14 1993-09-21 Kokusai Denshin Denwa Co., Ltd. Echo canceller
US5305309A (en) * 1989-12-06 1994-04-19 Fujitsu Limited Echo canceller
US5315585A (en) * 1992-05-15 1994-05-24 Kokusai Denshin Denwa Co., Ltd Echo canceller using two residual echoes
US5768308A (en) * 1994-12-19 1998-06-16 Northern Telecom Limited System for TDMA mobile-to-mobile VSELP codec bypass
US5796731A (en) * 1994-12-30 1998-08-18 Alcatel Nv Multiline PCM interface for signal processing

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FR2469044A1 (fr) 1979-08-30 1981-05-08 Thomson Csf Mat Tel Procede de traitement en temps partage de signaux numeriques et application a un annuleur d'echo autoadaptatif multiplexe
EP0177239A3 (fr) 1984-10-01 1987-06-03 AT&T Corp. Filtre adaptatif à compensation de voie signal
US5177734A (en) * 1988-05-02 1993-01-05 Itt Corporation Multirate wire line modem apparatus
US5005168A (en) * 1988-05-02 1991-04-02 Itt Corporation Multirate wire line modem apparatus
US5828696A (en) * 1995-03-30 1998-10-27 Lucent Technologies Inc. Timing recovery in a network-synchronized modem
US5991278A (en) * 1996-08-13 1999-11-23 Telogy Networks, Inc. Asymmetric modem communications system and method
US5790658A (en) * 1996-10-28 1998-08-04 Advanced Micro Devices, Inc. High performance echo canceller for high speed modem
US5995540A (en) * 1997-01-08 1999-11-30 Altocom, Inc. System and method for reducing processing requirements of modem during idle receive time
US6259680B1 (en) * 1997-10-01 2001-07-10 Adtran, Inc. Method and apparatus for echo cancellation
FI981091A (fi) 1998-05-15 1999-11-16 Nokia Networks Oy Menetelmä ja laitteisto kaiun poistamiseksi digitaalisessa matkaviestinjärjestelmässä
US6317419B1 (en) * 1998-06-10 2001-11-13 Conexant Systems, Inc. Method and apparatus for training an echo canceler in a PCM modem context
US6597732B1 (en) * 1999-01-14 2003-07-22 Eric Morgan Dowling High-speed modem with uplink remote-echo canceller
US6504865B1 (en) * 1999-02-24 2003-01-07 Altocom, Inc. Digital connection detection technique
US6580793B1 (en) * 1999-08-31 2003-06-17 Lucent Technologies Inc. Method and apparatus for echo cancellation with self-deactivation

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Publication number Priority date Publication date Assignee Title
US5305309A (en) * 1989-12-06 1994-04-19 Fujitsu Limited Echo canceller
US5247512A (en) * 1991-08-14 1993-09-21 Kokusai Denshin Denwa Co., Ltd. Echo canceller
US5315585A (en) * 1992-05-15 1994-05-24 Kokusai Denshin Denwa Co., Ltd Echo canceller using two residual echoes
US5768308A (en) * 1994-12-19 1998-06-16 Northern Telecom Limited System for TDMA mobile-to-mobile VSELP codec bypass
US5796731A (en) * 1994-12-30 1998-08-18 Alcatel Nv Multiline PCM interface for signal processing

Non-Patent Citations (1)

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Title
See also references of EP1157498A4 *

Also Published As

Publication number Publication date
JP2003531510A (ja) 2003-10-21
US6795405B1 (en) 2004-09-21
EP1157498A1 (fr) 2001-11-28
AU2388400A (en) 2001-07-09
EP1157498A4 (fr) 2002-04-17

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